In recent years, a social demand to weight reduction of vehicles such as automobiles has been increased more and more considering global environments. In order to cope with such demand, use of aluminum alloy materials of further reduced weight excellent in formability and paint bake hardenability has been increased, instead of iron and steel materials such as steel sheets, as a material for automobile panels, in particular, large body panels such as hoods, doors, roofs (outer panels and inner panels).
Among them, as panels such as outer panels (outer plates) and inner panels (inner plates) of panel structures, for example, hoods, fenders, doors, roofs and trunk lids of automobiles, use of Al—Mg—Si series AA or JIS 6000-series (hereinafter simply referred to as 6000-series) aluminum alloy plates has been under investigation as thin and high strength aluminum alloy plates.
The 6000-series aluminum alloy plates essentially contain Si and Mg and, particularly, excess Si 6000-series aluminum alloys have a composition with a Si/Mg mass ratio of 1 or greater and have excellent aging hardenability. Accordingly, they ensure formability by reduction of a yield strength in press forming and bending and have paint bake hardenability in which they are age-hardened by heating during artificial aging (hardening) such as paint baking treatment after press forming of the panel capable of increasing the yield strength and ensuring strength necessary as the panel (hereinafter also referred to as bake hardenability=BH response, bake hardenability).
In contrast, as is well-known, outer panels of automobiles are manufactured by applying press forming such as stretch forming and bending in combination in press forming to aluminum alloy plates. For example, in a large outer panel such as a hood or a door, the plate is press formed, for example, by stretch forming into the shape of a formed product as an outer panel and then bonded with an inner panel by hem working (hemming), for example, flat hemming at the periphery of the outer panel to form a panel structure.
The 6000-series aluminum alloy is advantageous having excellent BH response but, on the contrary, involves a subject of having a natural aging property of deteriorating the formability to a panel, particularly, bendability since it is age-hardened to increase the strength after being held at a room temperature for several months after solution and quenching treatments. For example, when a 6000-series aluminum alloy plate is used to an automobile panel, the plate is usually left put to a room temperature for about 1 to 4 months (left at room temperature) after the solution and quenching treatments by an aluminum manufacturer (after production) and before press formation into a panel by an automobile manufacturer in which the plate is age hardened considerably (natural aging at room temperature). Particularly, in an outer panel subjected to severe bending, while the panel can be formed with no trouble just after production, it involves a problem, for example, of causing cracking during hem working after age hardening (natural aging).
Further, when such natural aging is large, this also results a problem that the BH response is lowered and the yield strength is not improved to a strength necessary as the panel even by heating during an artificial ageing (hardening) treatment such as a paint baking treatment of the panel after the press forming described above.
Accordingly, various proposals have been made so far for improving the BH response and suppressing the natural aging of the 6000-series aluminum alloys. For example, Patent Literature 1 proposes suppression of change of strength in the course after lapse of seven days to 90 days at a room temperature after production by changing the cooling rate stepwise in the solution and quenching treatments. Further, Patent Literature 2 proposes to obtain a BH response and a shape fixability by keeping the alloy at a temperature of 50 to 150° C. for 10 to 300 minutes within 60 minutes after the solution and quenching treatments. Further, a Patent Literature 3 proposes to obtain the BH response and the shape fixability by defining the cooling temperature at the first stage and the subsequent cooling rate in the solution and quenching treatment. Further, Patent Literature 4 proposes to improve the BH response by a heat treatment after the solution and quenching treatments.
Further, many methods of positively adding Sn as the component to suppress the natural aging and improve the bake hardening have been proposed, for example, by Patent Literatures 5 to 11. For example, the Patent Literature 5 proposes a method of providing suppression for the natural aging and bake hardening together by defining the relation of components between Mg and Si as: −2.0>4Mg-7Si, adding appropriate amount of Sn having an effect of suppressing aging change, and applying preliminary aging after the solution treatment. Further, the Patent Literature 6 proposes a method of defining the relation of components between Mg and Si as: −2.0≤4Mg-7Si≤1.0, adding Sn having an effect of suppressing aging change and Cu of improving the formability, and applying galvanization, thereby improving the formability, bake hardenability, and corrosion resistance.